Optical measurements of patterned structures

ABSTRACT

A method and a system for optical measuring in a structure having a pattern in the form of spaced-apart parallel elongated regions of optical properties different from that of spaces between said regions. The system comprises a broadband illuminator ( 8 ) for generating incident radiation, a spectrophotometer arrangement ( 30 ) for detecting a spectral response of the structure to the incident radiation, and an optical arrangement ( 2 ) for directing the incident light to the structure and collecting the response of the structure, said optical arrangement ( 2 ) comprising a numerical aperture ( 32 ) selectively limiting the range of at least one of light incidence or collecting angles in direction substantially perpendicular to longitudinal axes of said elongated regions of the pattern.

FIELD OF THE INVENTION

This invention is in the field of measurement techniques, and relates tooptical system and method for accurate measurement parameters of regularpatterned structures. The invention is particularly useful forcontrolling semiconductor manufacturing process.

BACKGROUND OF THE INVENTION

Lithography is widely used in various industrial applications, includingthe manufacture of integrated circuits, flat panel displays,micro-electro-mechanical systems, micro-optical systems etc. Generallyspeaking, the lithography process is used for producing a patternedstructure. During the manufacture of integrated circuits, asemiconductor wafer undergoes a sequence of lithography-etching steps toproduce a plurality of spaced-apart stacks, each formed by a pluralityof different layers having different optical properties. Eachlithography procedure applied to the wafer results in the pattern on theuppermost layer formed by a plurality of spaced-apart photoresistregions.

To assure the performance of the manufactured products, the applicationsof the kind specified above require an accurate control of thedimensions of sub-micron features of the obtained pattern When dealingwith wafers, the most frequently used dimensions are the layer thicknessand the so-called “critical dimension” (CD). CD is the smallesttransverse dimension of the developed photoresist, usually the thicknessof the finest lines and spaces between these lines. Since the topographyof the measured features is rarely an ideal square, additionalinformation found in the height profile, such as slopes, curves etc.,may also be valuable in order to improve the control of the fabricationprocess.

Several Optical CD (OCD) measurement techniques recently developed relyon imaging a certain test pattern in the form of diffraction gratings,which are placed in a special test area of the wafer. The gratings areilluminated by light (typically a laser beam), and the resultingdiffraction pattern is analyzed to determine the line width and profileof the gratings. These techniques utilize various methods aimed atamplifying tiny differences in the line-width to obtain macroscopiceffects that could be resolved by visible light, although the originaldifferences are more than two orders of magnitude below the wavelengthused.

Techniques of the other kind utilize scatterometric measurements, i.e.,measurements of the spectral characteristics of a sample. To this end,when dealing with wafers, a test pattern in the form of a grating isplaced in the scribe line between the chips. The measurement includesillumination of the grating with a beam of incident light anddetermining the diffraction efficiency of the grating under variousconditions. The diffraction efficiency is a complicated function of aline profile and of the measurement conditions, such as the lightwavelength angle of incidence, polarization and diffraction order ofcollected light, thus providing a wealth of data allowing the extractionof information about the line profile.

Techniques that utilize the principles of scatterometry and are aimed atthe characterization of three-dimensional grating structures anddetermination of line profiles have been disclosed, for example, in theU.S. Pat. Nos. 5,867,276 and 5,963,329. Broadband scatterometry consistsof the illumination of a sample with an incident light beam having abroad spectral composition and detecting a beam of light diffracted fromthe sample with a spectrometer to obtain spectrally-resolved diffractioncharacteristics of the sample for determining the parameters of thesample.

However, in the above patent, these documents do not describe anyspecific method of measurements, or the constructional and operationalprinciples of specific adjustment or optimization of numerical aperturefor illumination/detection optical systems.

SUMMARY OF THE INVENTION

It is accordingly a need in the art to improve the optical measurementson patterned structures by providing a novel method and system formeasurements in a sub-micron patterned structure to determine parametersof the structure utilizing the principles of scatterometry.

It is a still further feature of the present invention to provide asystem whose operation is fast enough, so that every wafer in theproduction line can be measured, allowing a closer control over theprocess.

It is a still further feature of the present invention to provide such asystem, which enables an adjustable per application measurementprocedure, thereby decrease the calculation session without affectingthe quality of the entire measurement.

The main idea of the present invention is based on the following. Themeasurement process is based on the detection of spectralcharacteristics of light diffracted from a patterned structure (e.g.line array or grid), and the determination of desired parameters of thestructure based on these characteristics. Diffraction characteristic arewell known to depend on the incidence angle (3-Dimensional). Two maincases should be discussed: (a) when the plane of incidence (a planeincluding both the incident beam and the normal to the surface) isparallel to the grid lines, and (b) when the plane of incidence isperpendicular to the grid lines. It is also a general fact that thedependence of the diffraction characteristics on the incidence angle issmaller in case (a) than in case (b). When measuring the diffractionsignature using a real physical measurement system the signature isalways an average over some finite range of incidence angles, determinedby the Numerical Aperture (NA) of the measuring system. When attemptingto interpret such a measurement using a rigorous model the incidenceangle range is a source of error, since the basic model is calculatedfor a unique incidence angle. It is however impossible to reduce the NAwithout limit since the amount of collected signal per unit time,determining the practical signal to noise ratio (SNR), decreases withdecreasing NA. It is therefore required to find an optimum NA, takinginto account both the requirement for a good SNR and the requirementthat the diffraction characteristics do not change significantly withinthe used incidence angle range. When using a large, symmetrical (e.g.round) NA, it is clear, following the above discussion, that theoff-axis components of type (b) are the major source of error. It istherefore suggested in this invention to selectively limit the range ofincidence angles along the direction perpendicular to the grid lines,keeping the range of incidence angles in the parallel direction as largeas possible in order to preserve the total signal and hence the SNR.

Thus, in accordance with one broad aspect of the present invention thereprovided is an optical measurement system for measuring in a structurehaving a pattern in the form of spaced-apart parallel elongated regionsof optical properties different from that of spaces between saidregions, the system comprising a broadband illuminator for generatingincident radiation, a spectrophotometer arrangement for detecting aspectral response of the structure to the incident radiation, and anoptical arrangement for directing the incident light to the structureand collecting the response of the structure. The optical arrangementcomprises a numerical aperture selectively limiting the range of atleast one of light incidence or collecting angles in directionsubstantially perpendicular to longitudinal axes of said elongatedregions of the pattern.

A metrology system for performing the measurements using this inventionwould therefore include a non-circular aperture (e.g. slit-like,elongated), placed in such a location along the optical axis that therange of incidence angles of the light impingent of the sample isnon-symmetrical. In accordance with one embodiment of the presentinvention, such an aperture may be placed between the objective lens andthe image plane. Alternatively the aperture may be placed in the backfocal point of the objective lens or in an optically equivalent locationas shown below.

The aperture is non-circular, providing imaging and/or illuminating ofthe structure with NA different in two orthogonal directionscorresponding to the grid layout. The NA is relatively large along thelines of the grid and relatively small in direction, perpendicular tothe lines.

According to another aspect of the current invention the NA isadjustable according to the application. Adjusting the NA is done usingthe simulated sensitivity of the spectrum to the incidence angle.Accordingly, the measurement system includes variable aperture(s),allowing the selection between several NA configurations. The bestembodiment of such a variable NA mechanism would include twoperpendicular slit apertures, where each can be either in the opticalpath or outside the optical path. By selecting which slits are in thesystem can select among 4 different NA on figurations to be chosenaccording to the application and the grating direction. It is furtherpossible to use variable width slits, increasing the number of possibleNA configurations. An alternative aperture design that will also supportselection of NA per application is a variable circular aperture, thatcan be realized either as a continuous mechanism (iris) or as a set offixed apertures that are being replaced by some mechanism.

The patterned structure comprises a plurality of spaced-apart stacks,each including different layers having different optical properties, thepattern being formed by spaced-apart regions of an uppermost layer. Thedetermination of the desired parameters utilizes a certain optical modelbased on some features of a patterned structure of the kind specified,and presents the dependence between various parameters of the structureand the spectral characteristics of light returned from such astructure.

There is thus provided according to one aspect of the present invention,a measurement system for determining a profile of a patterned structurecomprising a plurality of spaced-apart stacks, each including differentlayers having different optical properties, wherein the pattern isformed by spaced-apart regions of an uppermost layer, the systemcomprising an optical measurement channels, including an illuminationassembly and a collection-detection assembly, and a control unit coupledto output of the first and second collection-detection assemblies,wherein: p1 the illumination assembly produces incident light ofsubstantially broad wavelengths band normally directed onto thestructure, and the collection-detection assembly detects spectralcharacteristics of light specularly reflected from the structureand-generates measured data representative thereof,

-   -   said control unit is capable of analyzing the measured data        generated by the collection-detection assembly for determining        at least one parameter of the structure, and utilizing said at        least one parameter for determining the profile of the        structure, wherein the illumination and/or collection-detection        assembly includes non-circular numerical aperture providing        different solid angles of illuminating and/or collected        light-beam-relative to the structure.

According to another broad aspect of the present invention, there isprovided a method for measuring in a structure having a pattern in theform of spaced-apart parallel elongated regions of optical propertiesdifferent from that of spaces between said regions, the methodcomprising illuminating the structure with incident radiation anddetecting a spectral response of the structure, wherein at least one ofincident radiation propagating towards the structure and the radiationresponse propagating from the structure passes through numericalaperture selectively limiting the range of at least one of lightincidence or collecting angles in direction substantially perpendicularto longitudinal axes of said elongated regions of the pattern.

In other words, there is provided method for measuring in a patternedstructure to determine a profile of the structure, wherein the structurecomprises a plurality of spaced-apart stacks, each including differentlayers having different optical properties, the pattern being formed byspaced-apart regions of an uppermost layer, the method comprising thesteps of:

-   -   illuminating the structure by a broad wavelengths band of        incident light normally directed on said structure, detecting        spectral characteristics of light specularly reflected from the        structure and generating measured data representative thereof,    -   analyzing said measured data and determining at least one        parameter of the structure and utilizing said at least one        parameter for determining the profile of the structure, wherein        said illuminating and/or detecting is performed using different        values of numerical apertures along and perpendicularly to said        structure lines.

More specifically, the present invention is used for controlling alithography process used in the manufacture of semiconductor devices(wafers), and is therefore described below with respect to thisapplication.

Preferably, the spectrophotometer is provided with an aperture stopaccommodated in the optical path of the specular reflected lightcomponent. The diameter and shape of the aperture stop is setautomatically according to the direction of the grid of the measuredstructure and according to the sensitivity of the specific applicationto the angle of incidence.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of the main components of ameasurement system constructed according to one embodiment of theinvention;

FIG. 2 is a schematic illustration of the main components of ameasurement system constructed according to another embodiment of theinvention

FIG. 3 is a schematic illustration of a wafer structure;

FIG. 4 exemplifies the angles of light propagation within differentranges along different axis; and

FIG. 5 is a schematic illustration of Numerical Aperture orientationrelative to the grid layout.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a measurement system 1constructed and operated according to the invention for measuringparameters of a wafer W (constituting a patterned structure). The system1 may represent one of the working stations of a semiconductors'production tool or line (not shown), the wafers W progressing betweenupstream and downstream stations of the production line. The system 1includes a measurement unit 2, a support stage 4 for supporting thewafer W and a control unit 6. Also provided in the system 1, is a waferhandler, which is not specifically shown The wafer handler serves forloading/unloading wafers to and from the stage 4, and may include asuction means for holding the wafer. Generally speaking, the waferhandler and wafer stage serve together for receiving wafers from aprocessing tool (not shown here) and returning them to the same oranother processing tool and optionally for aligning them alongcoordinate axes (eg.,by rotating the handler), maintaining, placing in ameasuring position, etc.

As schematically illustrated in FIG. 1, an optical path of measurementunit 2 comprises a broad-band (white) light source 8, for example aXenon arc lamp, an optional optic fiber 10, a condenser 12, whichdirects the light beam 13 onto a beam splitter 14, a relay lens 16, anda translatable objective lens 18. Behind the beam splitter 14 arelocated a pinhole mirror 20, a relay lens 22 and a pixel-array detectors(e.g., CCD) 24. Behind the pinhole mirror 20 there are: another relaylens 26, a mirror 28 and a spectrophotometer 30. In order to performmeasurements over entire surface of the wafer W, the only the objectivelens 18 may be translated, parallel to the plane of the surface of thewafer W, typically along with some mirrors which are not functionalparts of the optical path, e.g. as disclosed in U.S. Pat. No. 5,517,312assigned to the assignee of the present application. An aperture stop 32is accommodated in the optical path of the system between the objectivelens 18 and the image plane adapted to set the effective numericalaperture of the measurement unit 2. Preferably, the aperture 32 has anon-circular (e.g. slit-like) form, providing different values ofeffective numerical aperture NA in two orthogonal directionscorresponding to the wafer's W layout. The pixel-array detectors (e.g.,CCD) 24 along with appropriate hardware and/or software form an imagingutility, determining the location of measurement sites and theorientation of pattern elements.

The aperture 32 may be also provided with and coupled to and operated byappropriate drive-units (not shown) in order to individually adjust theeffective numerical aperture in two orthogonal directions.

As schematically illustrated in FIG. 3 (not in a correct scale), thewafer W typically has a plurality of stacks formed by different layers,and presents a structure with a periodic pattern. The measurements areaimed at determining the profile of the pattern lines, namely formed byit's the uppermost (photoresist in the present example) layer L_(a),either prior to or after the etching procedure applied to the wafer. Theuppermost layer L_(a) has a pattern formed by spaced-apart regions L_(b)of developed photoresist (in the case of prior-to-etching situation).Usually, the smallest transverse dimension of the developed photoresistis called the “critical dimension” (CD), however other definition of theCD may be also applied. The CD of the developed photoresist determinesthe CD of the entire wafer (i.e., patterned structure).

The parameters of the profile of the pattern lines to be determined mayinclude the following:

-   -   height of the profile (i.e., the thickness of the photoresist);    -   critical dimensions at the bottom and top of the photoresist        region, respectively;    -   radius of the curvatures at the bottom and top of the        photoresist region, respectively; and    -   the period of grating.

The above is the example of a symmetrical line profile. In the case ofan asymmetrical profile, a so-called “tilt” should also be determined.

Turning back to FIG. 1, the measurement unit 2 includes such mainconstructional parts as illumination and collection-detectionassemblies.

The illumination assembly is mainly composed of the light source 8, thebeam splitter 14 and the objective and relay lenses 16 and 18 that aredriven by a suitable driver (not shown) for auto-focusing purposes. Thelight source 8 generates incident light 13 of a broad wavelength band.The beam splitter 14 serves for spatially separating incident andreturned light components. The aperture 32, of a non-circular form,providing different values of effective numerical aperture NA of theapparatus in two orthogonal directions corresponding to the wafer's Wlayout. Here, a rectangular or slit-like aperture, such as exemplifiedin FIGS. 4 a and 4 b provides the angles of light propagation withindifferent ranges along its axis. Preferably, the NA_(∥) along grid linesis about 0.2 and the NA_(⊥) in direction perpendicular to the grid lineshas been reduced to 0.05 and even less. To this end, the light componentalong the grid lines propagates within a solid angle φ of about 20°(i.e., within a range of about ±10 from the normal) and light componentperpendicular to the grid lines propagates within a solid angle θ about10° in the present example. In this case, the intensity of illuminatinglight is reduced by factor 4 and is still sufficient for performingmeasurement with desired speed due to integration time of thepixel-array detectors 24 and spectrophotometer 30. It should be noted,that simple reduction of the numerical aperture from 0.2 to 0.05 willcase decreasing intensity of illuminating light by factor 16, which maybe un-sufficient for performing high speed measurements.

The collection-detection assembly also includes a spectrophotometricdetector 30 and a beam splitter 20 in the form of a pinhole mirror, thepurpose of which will be explained further below. The incident light 13normally impinges onto the wafer W, and light specularly reflected(normal “0” order) is collected and directed towards the detector 30, ina manner described further below.

It should be noted, although not specifically shown, that optical fibersmay be used for directing light components ensuing from the pinholemirror 20 to the detector 30.

Hence, the detectors could be mounted at any suitable locationAdditionally, a suitable drive assembly may be is provided for rotatingor/and moving the stage 4 within the X-Y plane or/and moving therespective optical elements, thereby enabling the measurements atdifferent locations on the wafer. The system 1 could be provided with adynamic auto-focusing assembly enabling high-speed measurements.

The construction and operation of a measurement system including thezero order detection spectrophotometer (measurement channel 2 ) and theimaging channel 26 is disclosed in U.S. Pat. No. 6,045,433 assigned tothe assignee of the present application. This document is thereforeincorporated herein by reference with respect to this specific example.

The pinhole mirror 20 separates a central part (about 20 μm) of lightspecularly reflected from the illuminated spot and collected by the lens18, and allows its propagation towards the spectrophotometric detector30. A periphery part of light beam is reflected from the mirror 20towards the imaging detector 24. As a result a measurement area,considered in the spectrophotometric detector 30, presents a 20 μm“dark” central region in a 1 mm×1 mm quadrant of the field of view ofCCD. This approach enables to locate the measurement area in the entireilluminated spot defined by the field of view of the CCD.

The outputs of the spectrophotometric detector 30 and the imagingdetector 24 are coupled to the control unit 5. The control unit 5 ittypically a computer device having a memory for storing reference data(libraries), one or more processor for analyzing data coming from thedetectors and controlling all the operations of the measurement system 1including driver(s), light sources, power supply, interface, etc. Thecontrol unit 5 also displays the measurement results. The processor isoperated by suitable image processing and pattern recognition software,capable of both global and site-to-site alignment The alignmenttechnique based on the features of the pattern is disclosed in U.S. Pat.Nos. 5,682,242 and 5,867,590, both assigned to the assignee of thepresent application.

Thus, the control unit 5 is capable of locating and processingmeasurements. The analysis of the measured data could be used forestablishing feedback closed-loop control of a corresponding processingtool, as will be described further below.

The measurement system according to the invention is designed so as toallow integrated optical measurement of CDs as well as other parametersof the wafer's profile. The operation of the system is based on themeasurement of the diffraction efficiency spectrum from the grating onthe wafer. The grating is any periodic structure composed of featureswhose parameters should be measured, e.g. minimal line-width, throughholes, etc. Due to the periodic structure, the diffraction from thefeatures on the wafer is limited to a discrete number of angles(diffraction orders), as governed by the diffraction equation:${\sin\quad\Theta_{r}} = {{\sin\quad\Theta_{i}} + {n\frac{\lambda}{d}}}$where Θ_(l) is the incidence angle, Θ_(r) is the reflected angle, λ isthe wavelength, d is the grating period and n is the order number (n=0being the specular reflection).

It should be noted that the measured gratings could be either anintegral part of the operative portion of the wafer (“patterned area”),or a test-pattern located in the non-operative portion (“margin area”)of the wafer. Such small test structures which are typically smallerthan 40 μm×40 μm are measured using a focusing lens.

Reference is made to FIG. 2 illustrating a measurement system 100,constructed and operated according to another embodiment of theinvention. To facilitate understanding, same reference numbers are usedfor identifying those components, which are identical in the system 1.In the system 100, a non-circular (elongated) aperture 132 is positionedin a plane located between a condenser lens 12 and a beam splitter 14and determines the effective numerical aperture of the illuminationassembly. Similarly to the aperture 32 the system of FIG. 1, itdetermines solid angle of illumination, having different values alongtwo mutually perpendicular axis. Alternatively or in addition, to thatconfiguration, a non-circular (elongated) aperture shown by dashed lines(designated 132′) may be positioned in a plane located between a relaylens 26 and a spectrophotometric detector 30.

The operation of the measurement system according to the invention willnow be described. Setup of the measurement includes the following twostages:

(1) Definition by the user of a profile model to be used and ranges foreach parameter of the selected model. Additionally, knowledge about allthe layers in the wafer and their optical properties, and any additionalrelevant information concerning the product (wafer) to be measuredand/or the measurement conditions is desired for defining themeasurement sites.

(2) Preparation of a library of spectra (reference data) correspondingto the possible profiles of the grid. The reference data may include setof spectra corresponding to the all or only part of the possibleprofiles. Each spectrum in the library gives the diffraction efficiencyfor a given profile of the grating, given polarization, given valuesnumerical aperture of the system, etc. The calculation is made using theknown Rigorous Couple Wave Analysis method (RCWT), modal methods, or bya hybrid method containing parts of both previous methods.

In accordance with the present invention, a novelty method and systempreferably employs non-circular aperture, providing on the one handsufficient signal and on the other hand simplifying the interpretationmode by allowing using a minimal number of incidence angles duringinterpretation. Thus, the calculation time of set up mode may besufficiently decreased without lose in accuracy. Additionally, in caseof using single angle of 0° in calculations, symmetric RCWT may beapplied, effecting in accelerating the calculation speed by a factor of3-5.

The preparation of the library may be made in one or more stages. Forexample, the following scheme may be used:

-   -   (1) Initially, the spectra corresponding to a small number of        profiles only are calculated, sparsely sampling the whole        multi-dimensional space of possible profiles.    -   (2) At this point, several measurements are taken and analyzed        using the initial library. Average values of the desired        parameters are determined, describing an average profile of the        wafer.    -   (3) A sub-space of possible profiles is defined around the        average profile. The sub-space is sampled with the required        (final) resolution, and the spectra of all profiles in the        sub-space are calculated.    -   (4) The rest of the profile space is divided into sub-spaces        with increasing, distance in the parameter space from the        average measurement    -   (5) These sub-spaces are consecutively sampled and their        corresponding spectra are calculated until the whole parameter        space is calculated in the final resolution.

Additionally, in distinction to alternative techniques, in which thesystem has no independent ability to prepare a library on site, theabove scheme advantageously has the issue of handling variations inoptical constants. It is well possible that over time, the opticalconstants of some layers will change slightly. This change could resultfrom lot-to-lot variations due to photoresist properties changes (e.g.composition) or slight changes in process conditions (e.g. temperature,humidity and process time). The chemical producer may disregard suchchanges since they are not supposed to have any direct effect on theprocess (e.g. changes in the optical constants of photoresist atwavelengths different than the exposure wavelength). On the other hand,any change in the optical constants of the measured layers willobviously have an effect on the measurement with the system 1. In orderto avoid this problem, the system has to monitor on a continuous basisthe optical constants of the layers, and, in case when them deviatesignificantly from the constants used for the calculation of thelibrary, the library has to be rebuilt. If the changes in the opticalconstants are sufficiently smooth, a system with on-board computationalpower will be able to follow the changes without a significantdeterioration in the measurement accuracy. Obviously, any technique thatrelies on external computational power will be disadvantageous in thescenario.

Measurement Procedure

Usually, the optical measurements are carried out on predetermined siteson the wafer, each containing a known layer stack, after performingwafer alignment utilizing a so called “alignment feature”. In otherwords, the knowledge of the layers' materials and thicknesses in thestack undergoing measurements, and the location of the alignmentfeature, are two inherent conditions, constituting the so-called “recipedesign”, for performing the optical measurements. The preparation of arecipe design associated with a specific article undergoingmeasurements. The term “recipe design” used herein signifies a computerfile containing the full information required to characterize a specificsite of the article. The site includes a stack of different layers,which may and may not include different locally adjacent sub-stacks(features of the pattern). The information contained in the recipedesign thus includes data indicative of the layers thickness, materialsand geometric details (e.g., wafer's features dimensions), opticalmodel(s) to be used for measurements in this site and interpretationdata (algorithms), die size, alignment feature location, etc.Additionally, in accordance with the present invention, differenteffective numerical apertures could be applied in the step of recipedesign and this information also included in the recipe design. Duringthe step of recipe design the effective numerical aperture could beadjusted for providing best result (sensitivity to specific profilefeatures, SNR, calculation time, etc.) of measurements. Recipe designmay be performed using the same measurement system that is used foractual measurements or any other appropriate tools. The sensitivity/bestperformance test could be also performed by simulation using appropriateoptical models. This recipe design, once prepared, can be used formeasuring one or more sites in the article to be measured.

Step 1. Alignment of the wafer W is performed by the wafer handler andwafer stage, so as to provide the correct position and orientation ofthe wafer W with respect to the measurement system 1. Alignment iscontrolled by feedback from position and angle sensors typicallyprovided in the measurement system, as well as from the imaging detector24. The alignment procedure is a very important stage of the entiremeasurement process, since diffraction efficiency is also a function ofthe angles between the incidence beam, normal to the wafer's surface andthe direction of the grating.

Step 2. The first measurement site is found This is implemented byproviding a relative displacement between the objective lens (andpossibly other optical elements) and the wafer along two mutuallyperpendicular axes within a plane parallel to the wafer's surface. Forthis purpose, feedback from images of some parts of the wafer acquiredby the imaging detector 24 can be used.

Step 3. In accordance with the present invention, the wafer W isoriented with respect to the measurement system 1 in such way, that gridlines in the test site are oriented along larger effective numericalaperture, as shown in FIG. 5. Alignment is controlled by feedback fromthe imaging detector 24 and may be additionally controlled by feedbackfrom spectrophotometric detector 30.

Step 4. Measurement of the reflection efficiency spectra is carried outat the normal incidence with the measurement system 1. Thesemeasurements usually are applied to the pre-determined sites S (seeabove) that include gratings, namely the sites where the measurementspot covers several photoresist regions L_(b) (FIG. 2). Thus, themeasurements can be taken from one or more, grating structures permeasured die, where different gratings may have different line/spaceratios in order to simulate different conditions of the controlledprocess.

In accordance with another embodiment of the present invention, theeffective numerical aperture with reduced value normally to direction ofgrating lines could be applied for step of recipe design only.Measurements in accordance with above described step 4 could beperformed with symmetric effective numerical aperture, or with effectivenumerical aperture providing best result (sensitivity to specificprofile features, SNR, calculation time, etc.) of measurements. In thatcase, the effective numerical aperture is adjusted to the value thatprovides the best measurement throughput for each or selectedmeasurement site S (specific feature).

Simulations performed by the applicant (using TM polarizations) showthat spectra are very sensitive to the solid angle ofillumination/collection defined by the value of effective numericalaperture having orientation normal to direction of grating lines. Also,it is at least order of magnitude less sensitive to the solid angle ofillumination/collection defined by the value of effective numericalaperture having orientation coincide with direction of grating lines.Thus it is possible to reduce the ‘effective Numerical Aperture’ just byits decreasing or adjusting in the orientation normal to direction ofgrating lines. Such scheme of adjusting the numerical aperture is muchbetter than simple of reducing the numerical aperture by reducing theradius of the shutter because more light is allowed on the site, thusshortening signal build-up (integration) time and increasing throughput

It should be noted that the direction of the slit is directly linked tothe direction of the grating. Thus, if grating sites with differentorientations are to be measured, the slit needs to be orientedaccordingly. The direction of the slit is unrelated to the direction ofpolarization.

It should also be noted that data indicative of the wafer's profilecould be used for adjusting the parameters of an etching tool prior toits application to the measured wafer or the next coming wafer, i.e.,for feed-forward purposes. Alternatively or additionally, themeasurement system can be used for post-etching measurement.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore exemplified without departing from its scopedefined in and by the appended claims.

1. An optical measurement system for measuring in a structure having apattern in the form of spaced-apart parallel elongated regions ofoptical properties different from that of spaces between said regions,the system comprising a broadband illuminator for generating incidentradiation, a spectrophotometer arrangement for detecting a spectralresponse of the structure to the incident radiation, and an opticalarrangement for directing the incident light to the structure andcollecting the response of the structure, said optical arrangementcomprising a numerical aperture selectively limiting the range of atleast one of light incidence or collecting angles in directionsubstantially perpendicular to longitudinal axes of said elongatedregions of the pattern.
 2. The system of claim 1 wherein said at leastone of said light incidence or collecting angles being limited by anglenot exceeds about 10°.
 3. The system of claim 2 wherein at least one ofthe light incidence or collecting angles in direction substantiallycoincide with the longitudinal axes of said elongated regions of thepattern being limited by angle not exceeds about 20°.
 4. The system ofclaim 1 wherein the value of said limiting numerical aperture does notexceeds about 0.05.
 5. The system of claim 4, wherein the value of thenumerical aperture in direction substantially coincide with thelongitudinal axes of said elongated regions of the pattern does notexceeds about 0.2.
 6. The system of claim 1 wherein said opticalarrangement for directing the incident light to the structure andcollecting the response of the structure further comprising an objectivelens and said limiting numerical aperture located between said objectivelens and an image plane.
 7. The system of claim 1 wherein said opticalarrangement for directing the incident light to the structure andcollecting the response of the structure further comprising an objectivelens and said limiting numerical aperture located in the back focalpoint of said objective lens or its optically equivalent location. 8.The system of claim 1 wherein said optical arrangement for directing theincident light to the structure and collecting the response of thestructure further comprising an illumination condenser and a beamsplitter for splitting said incident light and collected response andsaid limiting numerical aperture located between said condenser lens andsaid beam splitter.
 9. The system of claim 1 wherein said opticalarrangement for directing the incident light to the structure andcollecting the response of the structure further comprising a relay lenslocated before the spectrophotometer arrangement and said limitingnumerical aperture located between said relay lens and saidspectrophotometer arrangement
 10. The system of claim 1 furthercomprising imaging utility for determining location and orientation ofpattern of interest.
 11. The system of claim 1 wherein said limitingnumerical aperture is adjustable.
 12. The system of claim 1 wherein saidlimiting numerical aperture is of slit-like shape.
 13. A method formeasuring in a structure having a pattern in the form of spaced-apartparallel elongated regions of optical properties different from that ofspaces between said regions, the method comprising illuminating thestructure with incident radiation and detecting a spectral response ofthe structure, wherein at least one of incident radiation propagatingtowards the structure and the radiation response propagating from thestructure passes through numerical aperture selectively limiting therange of at least one of light incidence or collecting angles indirection substantially perpendicular to longitudinal axes of saidelongated regions of the pattern.
 14. The method of claim 13 whereinsaid at least one of said light incidence or collecting angles beinglimited by angle not exceeds about 10°.
 15. The method of claim 14wherein at least one of the light incidence or collecting angles indirection substantially coincide with the longitudinal axes of saidelongated regions of the pattern being limited by angle not exceedsabout 20°.
 16. The method of claim 13 wherein the value of said limitingnumerical aperture does not exceeds about 0.05.
 17. The method of claim16 wherein the value of the numerical aperture in directionsubstantially coincide with the longitudinal axes of said-elongatedregions of the pattern does not exceeds about 0.2.
 18. The method ofclaim 13 wherein the orientation of the pattern is determined byapplying imaging of said pattern.
 19. The method of claim 13 whereinsaid range of limiting of at least one of light incidence or collectingangles in direction substantially perpendicular to the longitudinal axesof said elongated regions of the pattern being determined bypredetermined value of signal to noise ratio.
 20. The method of claim 13further comprising calculating at least one of optical or geometricalparameters of said pattern and wherein said range of limiting of atleast one of light incidence or collecting angles in directionsubstantially perpendicular to the longitudinal axes of said elongatedregions of the pattern being determined by pre-determined value ofcalculating time.
 21. The method of claim 12 wherein said calculating atleast one of optical or geometrical parameters of said pattern performsby applying a RCWT.
 22. The method of claim 13 further comprising a stepof recipe design including determination of optimal value of limiting ofat least one of light incidence or collecting angles in directionsubstantially perpendicular to the longitudinal axes of said elongatedregions of the pattern in specific site of the article.